92 THE VOYAGE OF H.M.S. CHALLENGER. 



There are two completely free abdominal segments, the third being to a large extent 

 fused with the caudal shield; these segments decrease in size from before backwards; 

 the first is as long as any one of the first four thoracic segments, the second is not much 

 shorter, while the third is not more than half as long as the first. 



The first abdominal segment is divided into two distinct portions by a circumferential 

 furrow; the anterior portion is considerably shorter, and bears on the ventral side of the 

 segment a short spine on either side, the posterior half of the segment is smooth and 

 convex and has a pair of long, somewhat backward]}- curved spines, corresponding to the 

 short ones in front, and situated just in front of the smooth area against which tin' 

 anterior extremities of the uropoda play ; this segment as well as the next has on each 

 side a short lateral spine. The third abdominal segment has on each side a long curved 

 spine situated at the posterolateral margin of the segment, just in front and on a line 

 with the flattened margin of the caudal shield. The caudal shield has a central convexity 

 bordered by a flattened area which commences just behind the curved spine of the 

 segment in front ; the lateral margin is serrate, especially posteriorly, and terminates in 

 a pointed extremity on either side which projects beyond the termination of the. central 

 portion of the caudal shield; the latter terminates in a short median spine. 



The antennules are short, not reaching much beyond the second joint (if the 

 antenna. 



The joints of the antennse (fig. 5) increase progressively in length, the fourth and the 

 fifth being, however, subequal ; the inferior surface of the second joint of each antenna 

 a very downwardly pointing spine, as in its near ally Arcturus comutus', the flagellum 

 consists of more than eighteen joints and is longer than the terminal joint of the organ. 



The anterior thoracic limbs are not markedly longer than the posterior. 



The proximal joints of the second, third, and fourth thoracic appendages (fig. 3) are 

 furnished with longish spines, one or two to each joint, like those upon the segments ; tie- 

 posterior thoracic limbs (fig. 4) have no such spine, with the exception of the last, which 

 have a single spine about half way along the proximal joint. The arrangement of the 

 spines upon the thoracic limbs is as follows: the proximal joint has a single spine in the 

 first two of these pairs of appendages and an additional one in the fourth pair of ambula- 

 tory limits ; the second and third joints have a single spine, which is not very long, close 

 to the articulation of the joint with the succeeding one. 



The free margins of the uropoda where they come into contact with each other are 

 serrate (fig. 2). 



Station 320, off Rio de la Plata, February 14, 1S7G ; hit. 37° 17' S., long. 53° 52' 

 W. : depth, GOO fathoms ; bottom temperature, 37°*2 F. ; green sand. 



